A number of approaches have been developed for treating coronary artery disease. In less severe cases, it is often sufficient to merely treat the symptoms with pharmaceuticals or to treat the underlying causes of the disease with lifestyle modification. In more severe cases, the coronary blockage can be treated endovascularly or percutaneously using techniques such as balloon angioplasty, atherectomy, laser ablation, stents, and the like.
In cases where these approaches have failed or are likely to fail, it is often necessary to perform a coronary artery bypass graft procedure (“coronary bypass procedure”). In this procedure, direct access to the heart is first achieved. This is usually done by opening the chest by median sternotomy and spreading the left and right rib cage apart. The pericardial sac is then opened to achieve direct access to the heart. Next, a blood vessel or vessels for use in the graft procedure are mobilized from the patient. This usually entails mobilizing either a mammary artery or a saphenous vein, although other graft vessels may also be used.
A heart-lung or cardiopulmonary bypass is then performed. This procedure usually entails arterial and venous cannulation, connecting the bloodstream to a cardiopulmonary bypass system, cooling the body to about 32 degrees Celsius, cross clamping the aorta, and cardioplegic perfusion of the coronary arteries to arrest and cool the heart to about 4 degrees Celsius.
The arrest or stoppage of the heart is generally carried out because the constant pumping motion of the beating heart makes surgery upon the heart difficult. Cooling the body protects the organs from ischemia (a condition in which a tissue or organ does not receive a sufficient supply of blood), reduces the cardiac output requirement, and increases the systemic vascular resistance, which helps maintain perfusion and reduces the cardiopulmonary circuit primary volume.
Once cardiac arrest is achieved, a graft (or grafts) is attached to the relevant portions of a coronary artery (or arteries) followed by weaning from the cardiopulmonary bypass, restarting the heart, and decannulation. Finally the chest is closed.
After arresting the heart, the heart muscle, or myocardium, is protected and supported so that it does not suffer cellular or nerve damage that would prevent the heart from working properly when it is started again. There are two important aspects to the process of myocardial protection: (1) reducing the oxygen demand of the heart muscle; and (2) adequately oxygenating the heart muscle and maintaining the proper chemical balance so that cellular damage does not occur. One common technique for doing so is known as cold cardioplegia.
During this procedure, the coronary arteries must be isolated to prevent reperfusion of the myocardium with warm oxygenated blood from the cardiopulmonary bypass system that would wash out the cardioplegic agent and prematurely start the heart beating again. The most common way to isolate the coronary arteries is by aortic cross clamping, which is normally implemented in the following fashion. Before stopping the heart, the patient is prepared by placement of an arterial cannula and a venous cannula, which are connected to the cardiopulmonary bypass system. The cardiopulmonary bypass system takes over the functions of the heart and the lungs of the patient by pumping and oxygenating the blood while the heart is stopped. Once the cardiopulmonary bypass system is connected and started, the ascending aorta can be cross-clamped to isolate the coronary arteries from the rest of the systemic arterial circulation. Then, cardioplegic arrest is induced by injecting 500-1000 cc of cardioplegic solution into the aortic root using a needle or cannula which pierces the wall of the ascending aorta upstream of the cross clamp.
Unfortunately, significant complications may result from such procedures. For example, application of an external cross-clamp to a calcified or atheromatous aorta may cause the release of emboli into the brachiocephalic, carotid or subclavian arteries with serious consequences such as strokes.
Systems have been proposed in which the aorta is occluded without cross clamping. For example, U.S. Pat. No. 5,957,879 describes systems that include an aortic occlusion device having a balloon to occlude the ascending aorta and a lumen to deliver cardioplegic fluid for arresting the patient's heart. The aortic occlusion device replaces the conventional external cross-clamp and is said to reduce the amount of displacement and distortion of the aorta. Nonetheless, distortion is not eliminated, and the risk of emboli release remains present.
Other complications can arise from the cardiopulmonary bypass system, which includes mechanical blood pumps, an oxygenator, a heat exchanger, blood reservoirs and filters, and several feet of tubing to transport the blood from the patient on the operating table to the heart-lung machine located nearby and back to the patient. Such systems can cause complications due to the exposure of blood to foreign surfaces, which result in the activation of virtually all the humoral and cellular components of the inflammatory response, as well as some of the slower reacting specific immune responses. Other complications from cardiopulmonary bypass include loss of red blood cells and platelets due to shear stress damage. In addition, cardiopulmonary bypass requires the use of an anticoagulant, such as heparin. This may, in turn, increase the risk of hemorrhage. Finally cardiopulmonary bypass sometimes necessitates giving additional blood to the patient. The additional blood, if from a source other than the patient, may expose the patient to blood-borne diseases.
Due to the risks noted above, others have attempted to perform a coronary artery bypass graft procedure without occluding the aorta and without cardiopulmonary bypass. For example, attempts have been made wherein surgery is performed on a beating heart. The technique of operating on the beating heart, however, is difficult, due to the rapid movement of the heart, and can at present only be applied to single vessel bypassing procedures. Moreover, partial aortic cross clamping is generally implemented, which can dislodge emboli.
In other reported procedures, surgeons have been experimenting with a technique that involves stopping or nearly stopping the heart and supporting circulation with a small pump positioned in the patient's vasculature (i.e., an intracorporeal pump). See, for example, M. S. Sweeney, “The Hemopump in 1997: A Clinical, Political, and Marketing Evolution”, Ann. Thorac. Surg., 1999, Vol. 68, pp. 761-3 in which a coronary bypass procedure is described that uses a Medtronic Hemopump® for circulatory support and the patient's own lungs from oxygenation. Esmolol, a short acting beta-blocker, was administered to make the heart more tranquil during surgery. The interior surface area of the Hemopump is greatly reduced relative to traditional cardiopulmonary bypass systems, reducing the complications of such surfaces.
Unfortunately, it can be difficult to provide adequate circulation with a pump of this type, increasing the risk of ischemia. Moreover, while many of the dangers associated with cardiopulmonary bypass systems are avoided, certain benefits of such a system are also lost. For example, hypothermia is no longer induced in the patient, which serves to lower oxygen demand and which induces vasoconstriction, supporting perfusion. Each of these effects serves to protect the organs from ischemic damage.
Still other techniques have been proposed in which the heart is stopped or nearly stopped (e.g., placed in a reversible, temporary heart block) by locally delivering drugs, such as beta-blockers. At the same time, the heart is continuously paced by external pacemaker stimulation. In this way, alternating periods of heartbeat and heart arrest (e.g., up to 15 seconds) can be established, providing the surgeon with short intervals in which he or she can work on a stilled heart without resorting to a pump for supporting circulation. One such system is the TRANSARREST system of Corvascular, Inc., Palo Alto, Calif. Still other methods are known in which surgery is facilitated by stopping or slowing the heart though electrical stimulation of the vagus nerve. See, e.g., U.S. Pat. Nos. 5,913,876 and 6,006,134.
Unfortunately, as in the above case wherein the Hemopump supports circulation, these techniques result in less than ideal circulation and do not provide a hypothermic effect, increasing the risk of ischemia.
Medical procedures are also known in which hypothermia is induced in a conscious or semiconscious person, for example, where hypothermia is induced in a stroke victim to reduce ischemic damage. However, in such patients, hypothermia activates the sympathetic nervous system, resulting in a significant norepinephrine response. Norepinephrine, in turn, binds to beta-receptor sites, including those in the heart, causing the heart to beat harder and more rapidly, frequently resulting in cardiac arrythmia and increasing the risk of myocardial ischemia. Norepinephrine also causes peripheral vasoconstriction, frustrating relief of patient discomfort, for example, by using heating blankets.